<!DOCTYPE html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<title>Title</title>
</head>
<body>

</body>
<script>
	// * undefined 代表变量没有赋值
	// let a;
	// var b;
	// console.log(a,b);// undefined undefined
	
	
	// * null 代表变量赋值，只是值为null
	let a = null;
	const b = null;
	var c = null;
	console.log(a===b,b===c);// true true

</script>
</html>